The present invention relates to an apparatus and a method for determining the projection geometry of X-ray apparatuses for recording radiographic projection images of objects and for representation of the objects, derived from this, with the aid of a reconstructive imaging method. In particular, the present invention relates to an apparatus for geometric calibration of mobile and stationary C-arc X-ray systems and of X-ray systems for tomosynthesis.
Both in the case of C-arc X-ray systems and in the case of X-ray systems for tomosynthesis, an investigation object is located in the beam path between an X-ray source and a detector which is located opposite the X-ray source. The X-ray radiation which is emitted from the X-ray source is directed at the detector, so that the X-ray radiation passes through an object which is located between the X-ray source and the detector. The X-ray radiation is attenuated along each projection line as a function of the local X-ray absorption characteristics of the investigation object, on each of the projection lines between the X-ray source and the detector, so that this results in a distribution of the X-ray intensity on the detector surface which reflects the different attenuation of the X-ray radiation by the investigation object in the projection direction. The radiation which is received by the detector is converted to digital signals in order to produce a projection image.
A number of projection images are recorded in different projection conditions both for 3D reconstruction and for tomosynthesis. In tomosynthesis, a number of projection images are reconstructed on a focus plane, or else different projection images are combined to form a partially three-dimensional image. Tomosynthesis can be carried out on different X-ray systems, also including C-arc X-ray systems. In the case of C-arc X-ray systems, the radiation is normally passed through the investigation object from different angles and, in consequence, the investigation object is reconstructed in the form of a three-dimensional image from the projection images obtained in this way. This method is generally referred to as 3D reconstruction.
In comparison to computer tomography, C-arc X-ray systems are distinguished by having very short measurement times. C-arc X-ray systems are thus used by preference particularly when using contrast means which remain in the investigation area for only a short time, for example in angiography. In comparison to mobile C-arc X-ray systems, stationary C-arc X-ray systems are generally distinguished by shorter measurement times and a longer focal length. Mobile C-arc X-ray systems can be moved to an operating table both during and after operations, so that they are used especially for intraoperative investigation purposes and for monitoring records after completion of an operation.
In order to make it possible to make reliable statements about an object being investigated, geometrically exact reconstruction of the image representation of the object being investigated, with adequate position resolution and with a low artefact level, from the recorded projection images is essential. For 3D reconstruction on C-arc X-ray systems, a number of projection images or X-ray images are recorded at equidistant or variable angle increments, with the overall angle of the orbital rotation of the X-ray source and the detector located opposite it typically being 180xc2x0 plus half the beam angle of the beam lobe emitted from the X-ray focus, together amounting to approximately 190xc2x0. A representation of high-contrast objects such as bones with sufficient position resolution and with a sufficiently low artefact level can be derived from only approximately 50 to 100 projection images. A greater number of X-ray images are required for better image quality of the isotropic 3D data cube which is derived from the projection images. In order, for example, to achieve better contrast resolution with a lower noise level and a lower artefact level, up to 200 records or more may be required in some cases. The precise number of X-ray images required for an investigation is not fixed, but depends on the respective specific requirements for each investigation.
The calculation of the data cube is dependent on knowledge of the projection geometry of each individual projection image. Apart from the physical design of the system, the individual projection geometries of a system are also influenced by the mechanical tolerances in manufacture and, in particular, by deforming influences resulting from the force of gravity on projecting parts of the system.
The recording geometry and the projection for each individual projection image can be described by a projection matrix. The projection matrix can be defined by imaging a defined calibration phantom and by looking for the appropriate structure in the X-ray image for each specially distinguished point in the calibration phantom. In the simplest case, specially distinguished points such as these are in the form of small stainless steel balls in the calibration phantom. If a sufficient number of correspondences are found between specially distinguished points in the calibration phantom and their images in the X-ray image, so-called 2D-3D correspondences, a projection matrix can be defined for the specific recording geometry of that X-ray image. The projection matrix produced in this way contains all the necessary information to describe the imaging geometry completely.
The calibration phantom which is disclosed in U.S. Pat. No. 5,835,563 comprises a ring with a low X-ray absorption capability, to whose circumference markings with a high X-ray absorption capability are applied. The calibration phantom is positioned in the system before a measurement, such that its image occupies only a subregion of the 2D projection image. The rest of the image area is available for imaging the object which is actually to be reconstructed. The projection matrix which is defined by this calibration phantom, which is referred to as a marker ring, for the image subregion can be transferred to the entire image area, although this can result in inaccuracies in the reconstruction of the data cube.
The mechanical stability of both stationary and mobile modern C-arc X-ray systems ensures a reproducible imaging and recording geometry over a lengthy time period. A calibration phantom therefore no longer need be imaged simultaneously with the investigation object in an X-ray record. Instead of this, the calibration can be carried out at relatively long time intervals off-line, that is to say separately from the recordings of an investigation object. The image area of the X-ray system is thus completely available for the actual object of the investigation.
In order to achieve the reconstruction of the isotropic data cube with the best possible resolution and accuracy, it is advantageous to distribute the markers uniformly over the entire 3D reconstruction volume.
For this purpose, U.S. Pat. No. 5,442,674 proposes a calibration phantom in the form of a hollow cylinder composed of plexiglass, in whose surface balls with a high X-ray absorption capability are incorporated along a helical line. The diameter, pitch and length of the helical line are designed such that the balls are distributed over the entire image area of the projection. One or more specially distinguished balls are designed to be somewhat larger than the others and are used as geometric reference points, from which the other marking balls can be identified by counting.
The specially distinguished balls are arranged in the center or close to the center of the helical line, so that the individual balls in the phantom can be identified even when the outer ends of the helical line are not also being imaged. If the images of one or more specially distinguished balls are superimposed during the recording of an image at a specific angle with the images of adjacent balls, then this makes it more difficult to identify the individual balls. The same is also often true for the superimposition of the images of adjacent balls. If the edge areas of the cylinder are not also imaged in the X-ray image, then an unknown number of balls will be missing from the helical line in the image, so that it is impossible to identify the balls in the image itself by counting, starting from one specially distinguished ball. In some cases, the information which is required for identification can be obtained only with increased computation complexity and with the assistance of identification information from preceding or subsequent projection images.
The object of the present invention is therefore to specify a calibration phantom which allows direct identification of each marking of the calibration phantom in each projection image of a projection X-ray system for 3D reconstruction and tomosynthesis.
This object is achieved by a calibration phantom for determining at least one projection geometry for X-ray apparatuses which are designed for recording radiographic projection images of objects for a representation of the objects which is derived from this with the aid of a reconstructive imaging method, with the calibration phantom having a support in the form of a defined volume body or hollow body and markings, which are applied in a linear arrangement to the surface and/or within the support, with the markings having a first X-ray absorption capability and the support having a second X-ray absorption capability, which is different to the first X-ray absorption capability, the linear arrangement of the markings precluding superimposition of an image of a first marking with an image of a second marking for a sufficient number of successive markings for each projection condition, with the physical embodiment of a marking representing a value assignment, and with the value assignments for a sequence of successive markings forming code information, and, furthermore, with a first number of successive markings forming a first type of code information such that each code information item of the first type occurs once, and only once, in both sequence directions of the markings, and is thus unique.
The object of the invention is furthermore achieved by a method for determining at least one projection geometry for an X-ray apparatus which is designed using a calibration phantom according to the invention for recording radiographic projection images of objects for a representation of the objects which is derived from this with the aid of a reconstructive imaging method, with the method having the following steps: the calibration phantom is placed in the projection area between the X-ray source and the detector, the projection images are recorded in different projection conditions, the position of each image of a marking in each projection image is determined, the code information is extracted from the images of the markings, that marking which causes the image is identified for each image of a marking with the aid of the extracted code information, and the parameters of the projection geometry are calculated from the association between the images of the markings and the position of the markings which cause them.
The above object is furthermore achieved by a calibration device for determining at least one projection geometry of an X-ray apparatus which is designed for recording radiographic projection images of objects for a representation of the objects which is derived from this with the aid of a reconstructive imaging method, with the calibration device having a calibration phantom according to the invention, a holding device for placing the calibration phantom in the projection area of the X-ray apparatus, and an evaluation device for carrying out the method according to the invention for determining the projection geometry.
In comparison to calibration of the projection geometry of projection X-ray systems using a marker ring, the calibration phantom according to the invention offers the advantage that qualitatively more accurate projection matrices can be calculated, thus allowing more reliable 3D reconstruction. This considerably reduces the level of the artefacts in the 3D reconstructions. A further advantage is that the calibration phantom according to the invention is very simple to place in X-ray systems since it is irrelevant what part of the arrangement of markings is imaged in the projection image. The identity of each individual marking of any given segment element of the marking arrangement can advantageously be determined from each projection image. The identification of the reading direction from the code information of adjacent markings also allows rotated positioning of the calibration phantom.
Further advantageous embodiments of the invention are defined in the corresponding dependent claims.
The value assignments of a second number of successive markings advantageously form a second type of code information, with each of the code information items of the second type furthermore advantageously occurring once, and only once, in one of the two sequence directions of the markings, and thus being unique. This allows optimization of the calibration processes in that the length and form of different code information items can be matched to the respective requirements of a calibration process.
The markings preferably have two different physical embodiments for one binary value assignment, so that the value assignment advantageously corresponds to the binary number system used in digital evaluation systems. The physical extent of a marking preferably represents the value-assigning physical embodiment of the marking, with a marking furthermore advantageously being in the form of a body with a spherical surface. This firstly results in the definition of a simple identification criterion which can easily be implemented in automatic identification systems, while secondly ensuring that each projection image of a marking is independent of the projection direction.
In one preferred embodiment, the support for the calibration phantom is cylindrical, with the symmetry of the support corresponding to the symmetry of the sequence of projection images. Furthermore, the markings are advantageously arranged along a helical line, so that the individual markings are never superposed in the projection direction.
The markings are preferably so closely adjacent that at least one code information item of the first type is imaged in a radiographic projection image, thus producing complete information about the position of the calibration phantom in the X-ray apparatus in one projection image. The extent of the physical distribution of the markings may also go beyond the projection area, at least in a dimension transversely with respect to the projection direction, so that the edge area of the cylinder, for which the markings are imaged very close to one another, does not appear in the projection image.